There's a saying in the career world: "Dress for the job you want." You need to act like a manager before they'll make you one. You need to act like a CEO before the board is going to put you at the helm. In short, it means you need to prove yourself first and then you'll be rewarded.
The same is true for personal development. If you want to be happy, you need to start acting like a happy person. If you want to be fit, you need to start acting like a fit person. And once you prove it to yourself, you'll be rewarded.
But we find ourselves unwilling to play the part. It feels shameful and dishonest. How can I honestly pretend to act like a star athlete when my belly is hanging over my waistband.
The problem is that we're conditioned to believing we are our past. After all, we did just devour that bag of cookies and imprint our big ass in the couch while binging another season of the series. That isn't star athlete behavior, ergo I'm not a star athlete.
But if what we just did is who we are, then we're also what we'll do next. I'd even say that:
What we just did is who we were, what we do next is who we'll be.
If we want to become more we have to start acting like it.